Remove manual enable/disable of IPython formatters (cont.)

Updates #2525

Implements changes to the `use_latex` flag to the `init_printing()` method discussed in that thread:
- Renames `png`to `external`
- Drop `mathjax` option

In addition, this enables the text/latex rendering always when `init_printing()` is called. Prior to this PR, either `use_latex=True` or `use_latex=mathjax` would enable this, with previous changes from the previous PR, this would require `use_latex=True`. I can't see any reason for this to not be enabled always. This effectively changes `use_latex` to control how LaTeX is rendered to png, while text-based rendering of LaTeX will always be enabled. At present, I think this is only done by the notebook with MathJax.

If there is disagreement on this last point, I'd say to add a `text` option to `use_latex`, which would basically be what `mathjax` was previously (i.e. only enable the text/latex rendering).